We have a house in Seattle that is 60 years old with 6-8" of blown in cellulose insulation in the attic with an inch of rock wool under that. There is basic ventilation in the attic. The trouble we are facing it seems the insulation is holding the summer heat during days over 80 degrees and in the evening the house is warmer than outside. Is it possible we need to replace the old insulation??
UPDATE: This answer is too simplistic and lacks understanding of the true issue. See a superior, more complete answer here: Ridge vent conversion creating heat problem
The insulation is not holding heat (i.e. acting as a thermal mass). The likelier problem is that your attic itself is what's holding heat. Attic heat gain during the summer is primarily caused by sunshine. The sun hits your shingles and heats them up, and that heat radiates down into the attic. The darker the shingles, the more pronounced the effect.
As a result, an attic during the summer can easily reach temperatures 20-40 degrees higher than the ambient temperature due to this solar gain, even in your relatively cloudy climate. You can see for yourself by going into the attic around noon or 2 PM. It should be punishingly hot.
It's your insulation's job to prevent all that heat from getting through the ceiling and heating up the house. 6-8 inches of cellulose (R-3.5/inch) + 1 inch of rock wool (R-4/inch) works out to R-25 - R-32, which are numbers that are decent but not great for an attic. You need more! Blow more cellulose in there; 10 or 12 inches is a good start.
Another approach involves changing the color or material of your roof. The next time you have to redo it, if you choose light-colored shingles, or even light-colored metal, that will help tremendously to reflect the sun's rays, preventing a lot of that heat from even making it into the attic to begin with. But by the same token, this will hurt you in the winter, when that heat is actually helping to keep your attic warmer than the outside temperature. The degree of the penalty depends on how much of a heating load you have compared to your cooling load. Adding more insulation will help with both and is the recommended approach.
This is a little late, but if you really want to keep heating and cooling costs down then you need to insulate the roof in the attic space as well as above the ceiling, basically every surface area in the attic space should be well insulated. I've rented a house here in Southern Arizona that had a very heavily insulated attic space (so well insulated that it would be just barely warm in the space on a 110 degree day), well insulated walls and high quality windows and the cooling cost was 1/3 of what other houses that only had the ceiling insulated and cheaper windows. The fact is a very well insulated house will stay around the same temperature all the time much like a cave.